[1]} have suggested using \(Y-K>0.46(g-z)+0.82\)  (here g and z are AB magnitudes and Y and K are Vega magnitudes, see also [2]}) to efficiently separate quasars with redshifts
\(z<4\)  from stars in the Y-K versus \(g-z\) 
color-color diagram. We think that this may still be the case in the \(z\) -W1 versus \(g-z\) 
diagram because \(z\)  and W1 bands are close to Y and K bands respectively. In order to check this idea, we obtain a sample of normal stars and a
sample of late-type stars with both
SDSS DR7 spectroscopy and WISE data. Similar to what we did for SDSS-WISE quasars, we adopted
6\(^{\prime \prime }\)  as the offset between the SDSS and WISE positions for the star
and late-type star samples, and deleted a few duplicated WISE sources with relatively
larger offsets. In order to get a reliable quasar selection criterion, we also
include only the quasars and stars with magnitude uncertainties in the \(g\) , \(z\)  and W1 bands
less than 0.2 mag. Finally we adopt the SDSS-WISE samples of 37,535 quasars, 19,765
normal stars and 15,359 late-type stars to investigate the criterion of
separating quasars from stars. We note that the SDSS spectroscopically identified star sample
is very biased as many of them have similar optical colors to quasars, but we believe that
including more stars with very different optical colors from quasars will not affect our
quasar selection criterion because these stars should be well separated from quasars in our color-color diagram
than the stars with optical colors similar to quasars.
